1. Field of the Invention
This invention relates generally to communication systems and more particularly, it relates to metallic channel unit network for interconnecting a two-wire transmission path to a four-wire transmission path. In particular, the present invention is directed to a metallic channel unit network which is used in direct current (DC), low frequency, and voice frequency applications for interfacing between a channel bank multiplexer having a four-wire circuit and a customer station having a two-wire circuit.
2. Description of the Prior Art
As is generally well known, burglar and fire alarm companies have leased or rented two-wire transmission lines from telephone companies so as to communicate alarm signals from their alarm-detection equipment located at a customer's premise to alarm-monitoring equipment located in their remote monitoring office. Further, test signals may be sent from the alarm-monitoring equipment over the transmission lines to the alarm-detection equipment so as to check for routine maintenance or even troubleshooting. These two-wire transmission lines are generally a part of the same wire pairs in a large multi-conductor metallic cable furnished by the telephone companies over the past many years. Recently, the telephone companies have been pulling out these metallic cables and replacing them with a fibre optics transmission medium suitable for a digital carrier system so as to realize multifold orders of magnitude more circuits over a single cable, thereby reducing costs.
Unfortunately, the alarm signals generated by the burglar and fire alarm companies cannot be conveniently converted over to the digital carrier system. This is due to the fact that there are different alarm-signalling techniques and each would require a specially-designed channel unit in order to be compatible with the digital carrier system. While the telephone companies could design special equipment for use with each of the different alarm techniques, this is generally undesirable since it would be quite complex for administration as well as being relative high in cost.
In U.S. Pat. No. 4,852,160 to Frederick J. Kiko et al. issued on Jul. 25, 1989, there is disclosed a channel unit interface circuit for interconnecting a two-wire, bidirectional signal transmitting means and a four-wire transmitting means in a communication system which is suitable for use with substantially all of the different alarm-signalling techniques utilized by the various alarm companies for connecting their equipment to a channel bank coupled to the four-wire means. The interface circuit of the '160 patent simulates a transmission line or cable over a digital carrier system in the frequency range of DC to 3.5 KHz with an amplitude variation between +110 to -200 VDC. However, this patented interface circuit suffers from the disadvantages of having a large signalling capacitance and thus a slow speed of operation.
Accordingly, it would therefore be desirable to provide a metallic channel unit network for interconnecting a two-wire transmission path over a four-wire transmission path, with a relatively low signalling capacitance, a faster speed of operation, low power dissipation, and low power consumption. The present invention represents a significant improvement over the aforementioned U.S. Pat. No. 4,852,160, which is hereby incorporated by reference.